1. Field of the Invention
The present invention relates to an automotive engine hood formed by joining together an outer panel and an inner panel and, more particularly, to an automotive engine hood capable of effectively absorbing energy imposed thereon by collision.
2. Description of the Related Art
Pedestrian protecting means have been prescribed by law in recent years to protect pedestrians from automobile accidents. Pedestrian protecting ability of automotive engine hoods is a noticeable index of rating automotive engine hoods. The size of automotive engines has been increased with the progressively increasing the output power of automotive engines. Multiplication of functions of automotive engines requires a large engine room due to increased parts and structural members to be installed in an engine room. Consequently, a pedestrian-protective space underlying the engine hood has been reduced. Thus the development of automotive engine hood capable of effectively absorbing energy imposed thereon by collision even if only a small space is formed under the automotive engine hood is indispensable to cope with both requirements of sporty car design and requirements of pedestrian-protective performance.
An automotive engine hood proposed in Jpn. Pat. No. 3674918 meeting those requirements is a hollow structure formed by joining together an outer panel and an inner panel so as to form a space between the outer and the inner panel. Another previously proposed automotive engine hood includes an outer panel, and an inner panel provided with dimples of different depths to form a space between the outer and the inner panel.
FIGS. 14A, 14B, 14C, 15A, 15B, 15C, 15D, 15E, 16A, 16B and 16C show a known automotive engine hood. FIG. 14A is a perspective view of the known automotive engine hood, FIG. 14B is a sectional view taken on the line Y-Y in FIG. 14A, FIG. 14C is a sectional view taken on the line X-X in FIG. 14A, FIG. 15A is a graph showing the relation between acceleration and stroke at a part a of the automotive engine hood shown in FIG. 14A at the impact of the pedestrian's head on the automotive engine hood, FIG. 15B is a graph showing the variation of acceleration of the part a of the automotive engine hood shown in FIG. 14A with time at the impact of the pedestrian's head on the automotive engine hood, FIG. 15C is a sectional view taken on the line XId in FIG. 14A, FIG. 15D is a sectional view taken on the line XIc in FIG. 14A, FIG. 15E is a sectional view taken on the line XIe in FIG. 14A, FIG. 16A is a typical view typically illustrating the known automotive engine hood at the impact of the pedestrian's head on the automotive engine hood, FIG. 16B is a graph of assistance in explaining the relation between acceleration of a central part of the automotive engine hood at the impact of the pedestrian's head on the automotive engine hood and time, and FIG. 16C is a graph showing the relation between acceleration and stroke at the central part of the automotive engine hood shown in FIG. 16B at the impact of the pedestrian's head on the automotive engine hood.
Referring to FIGS. 14A to 14C, an automotive engine hood 121, which is a corrugated automotive engine hood, includes an outer panel 122 curved in predetermined curvatures, and a corrugated inner panel 123 having a depressed edge part 125 and a plurality of ridge-like beads 129 extending perpendicularly to the longitudinal axis of the vehicle. The automotive engine hood 121 is formed by joining the depressed edge part 125 of the inner panel 123 to the edge of the outer panel 122 by hemming. As shown in FIG. 14B, spaces 124 are defined by the outer panel 122, and the depressed edge part 125 and parts between the adjacent ridge-like beads 129 of the inner panel 123. Top walls of the ridge-like beads 129 are bonded to the inner surface of the outer panel 122 by bonding parts 127.
The pedestrian-protective performance of the automotive engine hood is evaluated by head injury criteria (hereinafter abbreviated to “HIC”) expressed by Expression (1).
                    HIC        =                                            (                                                t                  ⁢                                                                          ⁢                  2                                -                                  t                  ⁢                                                                          ⁢                  1                                            )                        ⁡                          [                                                1                  /                                      (                                                                  t                        ⁢                                                                                                  ⁢                        2                                            -                                              t                        ⁢                                                                                                  ⁢                        1                                                              )                                                  ⁢                                                                  ⁢                                                      ∫                                          t                      ⁢                                                                                          ⁢                      1                                                              t                      ⁢                                                                                          ⁢                      2                                                        ⁢                                      a                    ⁢                                                                                  ⁢                                          ⅆ                      t                                                                                  ]                                max          2.5                                    (        1        )            where a is three-axis composite acceleration in G of the center of gravity of the head, t1 and t2 are times when the value of HIC reaches a maximum, 0<t1<t2, and (t2−t1)=15 ms.
As shown in FIGS. 16B and 16C, accelerations of the pedestrian's head at the impact of the pedestrian's head on the automotive engine hood 121 are classified roughly into a primary impact acceleration caused by the impact of the head on the automotive engine hood 121 and a secondary impact acceleration caused by the collision of the automotive engine hood 121 with vehicle components including an engine installed in the engine room. The relation between the respective magnitudes of the primary impact acceleration and the secondary impact acceleration is dependent on the construction of the inner panel; that is, cone type automotive engine hoods and beam type automotive engine hoods, which differ from each other in the construction of their inner panels, differ from each other in the relation between the respective magnitudes of the primary impact acceleration and the secondary impact acceleration. Typically, the relation between the acceleration a and time t and the relation between the acceleration a and stroke S are represented by curves shown in FIGS. 16B and 16C, respectively.
On the other hand, the automotive engine hood needs to have requisite basic properties including tensile rigidity, dent resistance, bending rigidity and torsional rigidity. Tensile rigidity is necessary to suppress elastic deformation when a force is exerted on the automotive engine hood to wax the automotive engine hood or when the automotive engine hood is pressed to close the automotive engine hood. The tensile rigidity is dependent on the thickness and the Young's modulus of the outer panel and the positions of the joints between the outer and the inner panel, namely, the positions of the bonding parts 127. Dent resistance is necessary to suppress formation of dents resulting from plastic deformation caused by flying gravels and the like. The dent resistance of the automotive engine hood is dependent on the proof stress and the thickness of the outer panel. Bending rigidity is necessary to suppress the elastic deformation of the edge part of the automotive engine hood resulting from the application of a pulling force to lock the automotive engine hood and the exertion of the reactive forces of rubber cushions, the damper stay and rubber seals on the automotive engine hood. Bending rigidity is dependent on the young's moduli and the shapes of the edge part of the inner panel forming the edge part of the automotive engine hood and reinforcing members, namely, the geometrical moments of inertia of those parts and members. Torsional rigidity is dependent on the bending rigidity of the edge part of the automotive engine hood and the thickness of a central part of the inner panel.
The automotive engine hood is required to have those requisite basic properties and to exercise the pedestrian-protective performance. A limited space extends under the automotive engine hood and over the engine and parts installed in the engine room. Therefore, most automotive engine hoods made of materials and having thicknesses and shapes designed so as to have those basic properties do not exercise the requisite pedestrian-protective performance.
When the head impacts on the known automotive engine hood 121 covering a small space as shown in FIG. 14A, the magnitude and the duration of the secondary impact acceleration at secondary impact is greater than those of the primary impact acceleration at the primary impact as shown in FIGS. 16B and 16C. Therefore, the secondary impact acceleration affects adversely to the HIC expressed by Expression (1) and the level of HIC is not satisfactory. Edge part 125 of the automotive engine hood 121 is required to have a high bending rigidity and hence, end parts of the ridge-like beads 129 of the corrugated inner panel 123 in the edge part 125 of the automotive engine hood 121 need to have a large geometrical moment of inertia and a high tensile rigidity. Therefore, as shown in FIGS. 15C, 15D and 15E, parts of the automotive engine hood 121 corresponding to the end parts of the ridge-like beads 129 are hard to crush and deform. Consequently, the secondary impact acceleration is high as shown in FIGS. 15A and 15B and the pedestrian-protective performance of the automotive engine hood 121 is poor.
One of method of solving those problems in the automotive engine hood is to maintain or increase the amount of energy absorbed at the primary impact, and to reduce the acceleration at the secondary impact by reducing crushing load capable of crushing the automotive engine hood. Such a method can be achieved by forming the inclined end walls 129a of the ridge-like beads 129 in an easily deformable shape or by reducing the inclination α (FIG. 14C) of the inclined end walls 129a of the ridge-like beads 129. If the inclination α of the inclined end walls 129a of the ridge-like beads 129 is reduced simply, the distance between the edge of the outer panel and each of the bonding part 127 at the opposite ends of the row of the bonding parts 127 increases. Consequently, the automotive engine hood cannot have sufficient tensile rigidity and the elastic deformation of the outer panel 122 increases.